The present invention relates to a solid state image pickup device having a shutter function and by utilizing a static induction transistor as a solid state imaging element.
There has been widely used a solid state image pickup device comprising a charge coupled device or an MOS transistor. The present tendency is to replace these elements by a static induction transistor (hereinafter referred to as SIT element).
As an example, FIG. 1 shows a circuit arrangement of a solid state image pickup device constructed by arranging in a matrix a plurality of isolated gate SIT elements. FIG. 2 shows a waveform chart for explaining the operation of the isolated gate SIT arranged in a matrix as shown in FIG. 1. Respective SIT elements 1-1.1, 1-1.2, . . . , 1-2.1, 1-2.2, . . . are of an n channel SIT of normal OFF type and form a picture cell. Each picture cell receives light inputs and generates video signal outputs which are read out by an XY address system. As shown in FIG. 1, respective SIT elements 1-1.1, 1-1.2, . . . , 1-2.1, 1-2.2, . . . have source electrodes which are connected to respective vertical scanning lines 2-1, 2-2, . . . , drain electrodes which are connected to a reference voltage source (in the embodiment shown, grounded voltage) and gate electrodes which are connected to respective horizontal scanning lines 3-1, 3-2, . . . . The horizontal scanning lines 3-1, 3-2, . . . are successively selected by signals .phi.G.sub.1, .phi.G.sub.2, . . . generated from a horizontal register (not shown). The vertical scanning lines 2-1, 2-2, . . . are successively selected and connected to a readout signal line 5 through respective line selection switches 4-1, 4-2, . . . which are made conductive by signals .phi.S.sub.1, .phi.S.sub.2 . . . generated from a vertical register (not shown) during selected periods of one horizontal scanning line.
When the signal .phi.S.sub.1 having a duration equal to one line scanning period shown in FIG. 2a is supplied to the line selection switch 4-1 of the vertical scanning line 2-1 from the vertical register, the line selection switch 4-1 becomes conductive and source and drain electrodes of respective SIT elements 1-1.1, 1-1.2, . . . corresponding to respective picture cells in the horizontal direction are biased to a certain voltage during a one line scanning period. Under this condition when the signal .phi.G.sub.1 shown in FIG. 2c is supplied to the horizontal scanning line 3-1 from the horizontal register, the SIT element 1-1.1 is selected so that the current equivalent to the charges caused by light, stored in a gate capacitance C.sub.g of SIT element 1-1.1 and having an amount corresponding to an amount of incident light, flows through a load resister 7, the transistor switch 4-1, and the SIT element 1-1.1 from a power supply source 6. As a result of this the variable component of the voltage drop generated in the load resistor 7 is obtained at an output terminal 8 as an output voltage corresponding to light incident upon the SIT element 1-1.1. In this way, after output signals corresponding to light inputs for the vertical scanning line 2-1 are read out successively from respective SIT elements 1-1.1, 1-1.2, . . . the following vertical scanning line 2-2 is selected by the signal .phi.S.sub.2 shown in FIG. 2b from the vertical register so that signals .phi.G.sub.1, .phi.G.sub.2, . . . generated from the horizontal register shown in FIGS. 2c and 2d drive respective SIT elements 1-2.1, 1-2.2 . . . , source and drain electrodes of which are biased by the vertical scanning line 2-2 in the same manner as the above, and thus outputs of the SIT elements are successively read out. The same operations are successively repreated thereby obtaining required image pickup output.
In the conventional solid state image pickup device described above the signal reading out and signal resetting can simultaneously be performed for respective SIT elements constituting the picture cell so that only the shutter speed of a certain period determined by a scanning period from the time at which the given SIT element 1-1.1 is selected to the time at which the same SIT element 1-1.1 is again selected can be obtained. Under the condition that, for example, the line selection switch 4-1 is conductive by the signal .phi.S.sub.1, thereby supplying the voltage of the supply source 6 to the vertical scanning line 2-1, when the signal .phi.G.sub.1 is supplied to the horizontal scanning line 3-1 a part of charges stored in the gate capacitance C.sub.g corresponding to an amount of incident light for another SIT element 1-2.1 which is not selected on the same horizontal scanning line 3-1 is discharged through the source electrode thereof. In the television image pickup system SIT elements of about 500 lines are arranged so that the decreased amount of light charges reaches 20 to 30% of the initial signal amount. This decreased amount cannot be ignored and leads to uneven or blurred picture.
In the above solid state image pickup device constructed by arranging SIT elements each forming picture cell in a matrix only after reading out the signals of whole picture the picture cells can collectively be reset. In order to further increase the shutter speed an external mechanical shutter must be provided as an exposure control shutter.